The president's threat comes amid growing concerns over the fate of US-backed Kurdish fighters, which Turkey has long declared to be a terrorist group.

Kurdish forces have been critical partners in the campaign against ISIS. They fear that in the absence of US protection, Turkey will launch an assault against them.

The Syrian Democratic Forces (SDF) have been instrumental in the fight against the Islamic State, and currently hold hundreds of IS prisoners.

In comments to reporters Monday, US Secretary of State Mike Pompeo said the US remains committed to its Kurdish allies.

President Donald Trump took to Twitter Sunday in an effort to reassure Kurdish forces fighting the Islamic State in Syria that the US still has their backs.

"Starting the long overdue pullout from Syria while hitting the little remaining ISIS territorial caliphate hard, and from many directions. Will attack again from existing nearby base if it reforms," Trump tweeted. "Will devastate Turkey economically if they hit Kurds. Create 20 mile safe zone....."

A member of the Syrian Defense Forces removes an IS flag from a building in Raqqa, Syria in April 2017. Zohra Bensemra/Reuters

Concerns over the fate of US-backed Kurdish militias have grown since the president announced his plan to rapidly withdraw troops from the country. Turkey considers the Kurdish People's Protection Units (YPG) to be a terrorist group, and has previously pledged to drive them out.

The president's threat against Turkey, which is a NATO member, was highly unusual of the US. But it's also increasingly common for a president who's moved intimidation tactics from behind closed doors to Twitter.

The YPG form a large part of the Syrian Democratic Forces (SDF), a US-backed group that has been a potent ally in the fight against the Islamic State. In October 2017, after a year-long battle, SDF fighters ousted ISIS from Raqqa, Syria — considered the last stronghold of the territorial caliphate.

SDF fighters currently hold hundreds of ISIS prisoners in their custody, a number that continues to grow as the militia gains territory. The fate of these prisoners also hangs in the balance should Turkey launch attacks against the Kurds Few, if any, countries are willing to accept the prisoners; releasing them would potentially allow them to rejoin the Islamic State or other militias.

President Trump has not elaborated on his comments. On Monday, US Secretary of State Mike Pompeo told reporters in Riyadh the president's tweet is consistent with US goals.

"If we can get a space, call it a buffer zone ... if we can get the space and the security arrangements right, this will be a good thing for everyone in the region," he said, according to Associated Press.